An important consideration in the development of personal care products is the control of microorganism contamination. To ensure that a consumer-ready product is safe, an appropriate preservation strategy must be established for the development, manufacture and packaging of the formula, thus eliminating unacceptable levels of organisms such as pathogenic bacteria, fungi and mold that could grow and pose a health hazard to the consumer.

Key considerations for microorganism exclusion1 throughout a product’s life cycle include:

Creating a hostile environment for growth, which involves formula optimization and preservative compatibility;

Combining preservatives for broad-spectrum benefits;

Using contamination-free raw materials;

Sanitizing process equipment;

Effectively designing packaging to protect the formula and ensure closure as well as a protective barrier, especially during use; Adjusting the process temperature and duration, to control waterborne contamination; and

Using microbiologically acceptable process water.

It is also important to establish an appropriate written good manufacturing procedure (GMP) protocol for raw material specifications, storage and handling; contamination-free water; and sanitization of equipment—including internal safeguards and aseptic manufacturing operating procedures. An appropriate GMP protocol would include such aspects as:

To control contamination, the adequate cleaning of all areas of equipment prior to sanitization is an important first step. Cleaning in place (CIP) systems should be used wherever possible to facilitate a sterile environment prior to processing commencement. Also, high quality water should also be used to rinse cleansing ingredients from the equipment—this applies in the laboratory as well, to eliminate contamination that could produce a negative micro-test of prototype samples under evaluation. Sanitization should include steam heat (water above 100°C) and germicidal products (e.g., chlorine, cationic surfactants and alcohol).

Optimizing Preservation

In order to optimize the preservation of a personal care product, the preservative system should provide broad-spectrum activity, efficacy at low concentrations—i.e., the minimum inhibitory concentration (MIC)—and act at optimum pH levels. In addition, it should be soluble in both water and oil phases.

The preservative system must also consider the potential for microbial growth in the water content of a formulation and any nutrients imparted by the preservative itself. This can be accomplished by understanding the raw material composition of the formula, i.e., the presence of natural ingredients, proteins, etc., and by using preserved surfactants. Further considerations include the formula’s stability under process, handling, and storage conditions; color and odor contributions of the preservative to the formula; and the preservative’s overall compatibility with the intended system and incompatibilities with other ingredients that may deactivate or weaken its performance. Finally, the shelf life, safety, ease of handling and cost effectiveness of the material are additional considerations.2

In most cases, a robust microorganism-free product starts with a sound formulation strategy and should include, as a key component, the elimination of all microorganisms. Therefore, it is important for formulators to understand the interaction of each raw material in the formulation with the various preservative options available. An optimum preservation concentration will ensure sufficient preservation levels, i.e., the MIC, yet not cause skin sensitization or reactions with other ingredients in the formula.

Water Activity (aw) as a Control Strategy

One way to control microorganism contamination in personal care cosmetic formulations is through the determination and control of the available water in a formula, known as the water activity,3-5 which is expressed as the ratio of vapor pressure of the formula against pure water at 20°C. Formula vapor pressure is considered the relative humidity of the head space over a product. The rationale is that microorganisms need water and nutrients to survive and grow. Reducing the amount of bioavailable water will thus retard microbial growth.

Using water activity as a strategy, it is possible to limit the access of microorganisms to nutrients such as proteins through enhanced partitioning between the aqueous and oil phase. By calculating the amount of available water (aw), one can predict the probability that the formula is preserved sufficiently to suppress the growth of microbes. This will improve the effectiveness of the preservative—i.e., less preservative is needed for improved efficacy—and provide a more robust formula that is able to inhibit the ability of microorganisms to multiple.

Pathogenic bacteria require an aw value in the range of 0.86 to 0.99, with Gram-negative bacteria being the most tolerant of a low aw. Yeast and mold are resistant to even lower aw values, at less than 0.70, and spores even more so. To decrease water activity in a formula, several materials can be used: glycerin, sorbitol, glycols including propylene, butylene and polyethylene, sugars, salts and dextrin.

Regulations and Preservative Testing

To protect the consumer, most governmental agencies have implemented regulations to address what constitutes a sterile, contaminant-free, pathogen-free product that is safe for consumer use. Some countries regulate the approved or unapproved types of preservatives allowed and control their maximum use levels. Besides governmental test recommendations, most preservative suppliers have internally tested their products and offer valuable and current data about them; this should be consulted before using a material in a finished product.

Product quality must be maintained throughout the normal use of the product by the consumer. To test the robustness of a formula’s preservation system to ensure it is sterile and safe for consumer use, the preservative level of a formula typically is tested 24–48 hr after processing (PET and D-Value pass/fail), after 1 month’s storage at 50°C (D-Value pass/fail), and after 3 months’ storage at 40°C (PET and D-Value). In addition, the following simple preservation tests can be used to assure that a consumer product is adequately preserved.

Preservation Efficacy Testing (PET): Also known as challenge testing, PET basically examines whether a product can withstand consumer contamination, and whether any contamination has occurred during manufacturing. If the sanitary quality of raw ingredients and processing equipment is maintained, there should be no initial contamination in the finished product. Unfortunately there are a number of possible protocols6 for this type of test, and each consumer company has its own in-house modifications based on specific needs.

The US Pharmacopeia (USP) and Personal Care Products Council also have published variations that are widely recognized as reliable methods. It should be noted that an aerobic plate count (APC) does not indicate how well a preservative is working because the preserved substance may not have been subjected to a significant bacterial load; i.e., the preservative system was not stressed. Thus, simply because no bacteria were present in the plates after an APC, this does not indicate the preservative system was efficacious.

D-Value: Determines the preservative efficacy as rate of kill. This method is good for selecting the type and concentration of a preservative to be used. It is also used as a measurement to indicate whether a formula is adequately preserved. Within this test, a linear regression method is used and the D-Value measurement indicates the number of hours required for a 90% reduction of the microbes introduced. The percentage of reduction is company-specific and may vary.

To determine efficacy against bacteria in a leave-on product, the test typically is conducted after less than 4 hr, and in a rinse-off formula, less than 8 hr. For yeast and molds, the test typically is conducted for leave-on products after less than 6 hr and for rinse-offs, less than 28 hr. It is important to remember that every change to the formula, such as the water content/ratio, ingredients, chelator, humectants level, fragrance or level of fragrance, during development requires a preservation re-evaluation.

Preservative Considerations

As noted, in order to choose an effective preservative system, one must consider interactions with other raw materials as well as when to process the preservative, sourcing microorganism-free raw materials, and choosing appropriate processing equipment and packaging. Following, and as shown in Table 1, are some suggestions from experienced formulators to ensure adequate preservation and a sterile environment.

Add preservatives to the water phase: Even when a preservative is oil-soluble, add it to the water phase of a formula to improve preservative contact with the aqueous phase, and so that it is not partitioned in the water-oil interface. Also, when adding preservatives to the water phase, the formulator should be careful of the aqueous phase pH, especially with parabens; often the water phase is alkaline, which can hydrolyze the parabens. Finally, if possible, add some preservative after emulsification for o/w emulsions to provide better aqueous phase preservation.

Consider combinations of preservatives: To develop a broad-spectrum microorganism kill, consider combinations of preservatives. Broad-spectrum activity is an important attribute of a preservative system, whether the combination preservative system is a single component or multi-component system. Also, it is better to add an antifungal compound with an antibacterial agent for improved broad-spectrum activity since each has a specific and complimentary function. In addition, using auxiliary ingredients such as pentylene glycol, hexylene glycol, glyceryl caprate and caprylate, sodium anisate or sodium levulinate can enhance preservative activity in the aqueous phase or at the aqueous-oil interface.

Test the raw materials: With regard to raw materials, test and reduce microorganism presence in them to eliminate a potential strain on the preservation system in a formula.

Lower water activity, water partitioning: Lower the water activity and water partitioning of a preservative to prevent the migration of it away from the aqueous phase. This can be accomplished by increasing glycerin and other polyols above 5%; reducing the surface tension between the oil phase and water phase with functional siloxanes, particularly dimethicone polyethers and flurosilicones; and minimizing sources of energy for microbial growth (e.g., carbohydrates, anionic surfactants, proteins, and natural gums).

Optimize the pH of the aqueous phase: This provides optimum preservative system activity. Also, use preservatives at the appropriate pH for the product. For example, organic acids or organic acid combinations should not be used over a pH of 6.0.

Enhance preservation system activity: Use EDTA to increase the microbial cell wall permeability of Gram negative bacteria cell walls by complexing it with the magnesium in the cell wall. This enhances the penetration of preservatives into the cell to capture essential micronutrient metals (e.g., iron) and more effectively eliminates Gram-negative bacteria, particularly with quaternary ammonium compounds, parabens, sorbic acid, imidazolidinyl urea and DMDM hydantoin. Other examples of possible chelating agents include citric acid and sodium citrate, etridonic acid, pentasodium triphosphate, sodium gluconate, phytic acid and sodium phytate. It should be noted that all the ingredients listed here do not necessarily enhance preservation in all formulas. It is important to choose based on the given application. For example, nonionics inactivate rather than enhance parabens. Also, there has been some discussion that amphoterics could have a negative effect on preservative enhancement.

Consider other factors when choosing a material: These can include the material’s odor; for example, DMDM hydantoin can have a fishy smell, and phenoxyethanol a somewhat unpleasant scent. Also consider the preservative’s discoloration potential. For instance, clear shampoos can turn yellow in the presence of urea. Some natural botanical oils will also discolor a formula over time in the presence of preservatives; the formulator should check their synergistic stability before using them in a formulation. Finally, safety and allergenicity are inherent concerns with all bioactive materials.

Consider ingredients that can enhance preservatives: Ingredients that can effectively enhance preservatives include cationics (quaterniums), due to their high antimicrobial activity, and anionics such as fatty acid soaps, alkyl benzene sulfonates and palmitic acid salts, for their weak antibacterial effects. Amphoterics including lauryl betaine and N-lauroyl-DL-phenylalanine may demonstrate similar activities to some cationic derivatives; however, these are not well-substantiated.

Nonionics such as monoesters of fatty acids and alkyl glycosides can enhance preservatives at concentrations below the critical micelle concentration (CMC) by improving their solubility, since at low concentrations a low interfacial tension exists, which allows increased interaction between the preservative and the microbe present in the aqueous phase.

Natural ingredients as preservatives: Essential oils and fragrances have some antimicrobial properties but typically require high concentrations in order to be effective; examples are: clove, cumin, eucalyptus, lavender, lemon, thyme, sage, sandalwood, neem and tea tree oils. It should be noted that in the presence of some pure botanical oils, caprylyl glycol and other long chain glycols are known to interfere with viscosity by reducing it over time. They can also cause discoloration.

Raw material/preservative interactions to consider: It should be noted that UV sunscreens can deactivate formaldehyde-releasing preservatives, and surfactant micelles tend to capture and inhibit preservative activity. Preservatives can also be compromised in the presence of solid particles as they may absorb onto the surface of the particles and thus become unavailable within the aqueous phase. Examples of solid particles could include: talc, inorganic sunscreens, clays and pigments. Proteins and highly ethoxylated compounds (strong hydrogen binders) may also deactivate preservative activity.

At > 15%, silicones can also make a formulation more difficult to preserve due to the creation of a separate phase and the retardation of microbe killing during insult testing. This is due to surface tensions, and interferes with current microbial assays designed to measure preservation efficacy. Finally, heat may inactivate preservatives such as formaldehyde donors and isothiazolinones by accelerating hydrolysis and breaking them down. It can also degrade and dissipate those that are more volatile. Thus, heating formaldehyde donors to above 60°C is not recommended; in this case, it is instead suggested that they be added after emulsification, as noted above.

Conclusion

It should be cautioned that all preservatives have some level of allergenic potential because they are bioactive in nature. Preservative suppliers have the latest safety data on their preservatives and this should be consulted before using a preservative in a finished product. In addition, consumer marketers should ensure they have properly tested the safety of their products.

Much has been published about preservatives but the industry continues to run into issues with their formulation. The guidelines presented here are intended to assist formulators in minimizing potential issues. It is important for the formulating chemist to understand raw materials, why and when to use preservatives, and water activity for rapid screening of a formula’s robustness. In addition, it is crucial to remain diligent when transferring the formulation to the development engineer and processing plant.

Acknowledgements: The author wishes to extend a special thanks to John Garruto of Free Radical Technology and Susan Lindstrom of ISP Corporation for their support with this column.

Biography: Eric S. Abrutyn, TPC2 Advisors Ltd., Inc.

Eric S. Abrutyn is an active member of the Society of Cosmetic Chemists, a Cosmetics & Toiletries scientific advisory board member, and chairman of the Personal Care Products Council’s International Nomenclature Cosmetic Ingredient (INCI) Committee. Recently retired from Kao Brands, Abrutyn founded TPC2 Advisors Ltd., Inc., a personal care consultancy. He has more than 35 years of experience in the raw material supplier and skin and hair care manufacturer aspects of personal care.